This invention relates generally to fluid pumping, and more particularly to fuel injectors that include a free floating plunger that can be uncoupled from the tappet over a portion of its movement.
Conventional mechanically actuated fuel injectors include a tappet assembly having a plunger and tappet that are mechanically coupled to one another. One example of such a tappet assembly is taught in U.S. Pat. No. 4,531,672, issued to Smith on Jul. 30, 1985. Smith teaches a tappet and plunger that are mechanically coupled by a spring, thus allowing the plunger to retract with the tappet under the action of a tappet spring at the end of an injection event. While performance of tappet assemblies has been acceptable, problems associated with plunger scuffing and seizure, as well as cavitation, have caused engineers to search for improvements. For instance, if a plunger, or tappet, is misaligned within its guide bore, the outer surface of the component can become worn. Eventually, this scuffing can lead to plunger failure. In addition, in the event of a plunger seizure in a tappet assembly such as that taught in Smith, the tappet spring will be prevented from expanding, which will allow separation between valve train components and can cause major valve train and engine damage. Further, in fuel injectors using the tappet assembly design taught in Smith, the plunger is retracted by the upward movement of the tappet spring when the rocker arm moves upward and relieves the downward pressure exerted on the tappet. If fuel cannot refill the fuel pressurization chamber as quickly as the plunger retracts, the fuel passages can depressurize. This can produce cavitation bubbles which can wear away the various surfaces of the injector body and fuel passages when they collapse. Problems resulting from cavitation erosion can be a significant source of wear and failure in fuel systems.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention, a plunger and tappet assembly has a body. A movable tappet assembly is mounted on the body and has a first contact surface. A plunger, which is preferably ceramic, is positioned in the body and is movable a distance and has a second contact surface that is adjacent the first contact surface.
In another aspect of the present invention, a fuel injector has an injector body that defines a fuel inlet. A pumping assembly has a free floating plunger and a movable working element that is positioned at least partially in the injector body and has a first contact surface. The free floating plunger is movable a distance and has a second contact surface that is adjacent the first contact surface. A cavity is defined at least in part by the first contact surface and the second contact surface and is substantially fluidly isolated from the fuel inlet.
In yet another aspect of the present invention, a method of pumping fluid includes providing a device that has a body defining a fluid inlet and a fluid outlet. A pumping assembly that has a free floating plunger is movable between a retracted position and an advanced position and a working element is at least partially positioned in the body and has a first contact surface. An amount of fluid is displaced through the fluid outlet by pushing the plunger toward the advanced position with the working element. The plunger is retracted by applying a fluid pressure to the plunger. The working element is retracted at least in part with a mechanical device.